Global pandemic threat keeps increasing as distances keep decreasing

New diseases had been appearing throughout the history and will continue to appear in the future thanks to evolution and mutation.
Outbreaks of illnesses have decimated cities and countries1. But the affected territory kept increasing together with people increasingly moving between regions. In the 14th century, black plague came from Asia along the Silk Road to wipe out half of Europe2, but other continents were still quite isolated and therefore not affected. Now with planes continuously connecting the whole world, a deadly illness could infect the whole world population within a couple of days from outbreak.

A disease would pose an immense threat to humanity if it possessed the following four qualities at the same time:

Spreads easily so that an infected person is very likely to infect people around him. The easiest way for an illness is probably spreading through air so people can get infected even without direct contact. Flu is an example of a common disease that spreads easily, while HIV does not.

Has long incubation period while being infectious so that it goes undetected and is difficult to isolate. It is easy to isolate people with clear symptoms to prevent disease from spreading, but if the symptoms are hidden for a long time, the infected person could travel and have plenty opportunities infect others. Common cold will show within couple of days, ebola within couple of weeks and rabies several months3. The average time before a person realizes they have HIV is around 12 years 4. Disease is way more threatening if it is infectious before you show symptoms (like HIV), but weaker when it is not infectious during the incubation period (like ebola).

Cannot be cured. With progress in science, we have been able to develop vaccinations or cures for most diseases. There are however still many diseases for which we have been unable to find cure despite they have been around for long time, like cancer. It is likely that cure will eventually be found for any illness, but the question is whether it will be fast enough before the illness kills the people who might come up with the cure.

Is fatal. Almost everyone had a cold in their life, but fortunately cold is not fatal so we are still all alive.

So far illnesses that emerged in the recent history of interconnected world never met all of these criteria. For example AIDS meets 2), 3) and 4), but not 1).

With ongoing mutations, it is just a question of time when illness that meets all four criteria breaks out to wipe out most or all of us.